A technique for realizing an optical integrated circuit on which optical parts are integrated is desired like a transistor integrated circuit on which electronic parts are integrated. At the moment, an optical circuit is formed by connecting optical parts such as an optical switch, a wavelength filter, and a 3 dB coupler (optical coupler), via an optical waveguide, for example, an optical fiber. However, if multiple optical parts can be integrated on a small chip, volume, power consumption, and manufacturing cost of the optical circuit is dramatically reduced.
There are many techniques developed aiming to realize the optical integrated circuit. One of the techniques is photonic crystal technique. In a broad sense, a photonic crystal body or a photonic crystal are general terms for a structure with periodically changing refractive index. In this document, unless otherwise noted, “photonic crystal body” and “photonic crystal” are used as synonymous words.
The photonic crystal has various special optical characteristics due to the periodic structure of refractive index distribution. The most representative feature is Photonic Band Gap (PBG). Although light can be transmitted through the photonic crystals, if the periodic refractive index change in the photonic crystal is large enough, light in a certain specific frequency band cannot propagate in the photonic crystal. The frequency band (or wavelength band) of the light which can be transmitted through the photonic crystals is referred to as a photonic band. On the other hand, the band in which the light cannot be transmitted is referred to as a Photonic Band Gap (PBG), indicating a gap existing between the photonic bands. Multiple PBG may exist in different frequency bands. The photonic band divided by PBG may be referred to as a first band, a second band, and a third band etc. in the ascending order of frequency.
If a minute defect which destroys the periodic structure of the refractive index distribution (periodicity of the refractive index distribution) exists in a photonic crystal, light in the frequency of PBG is confined in the minute defect. In that case, only the light in the frequency corresponding to the size of the defect is confined, thus the photonic crystal functions as an optical cavity. Therefore, such photonic crystal can be used as a frequency (wavelength) filter.
If minute defects are continuously located in lines in a photonic crystal, and a line defect is formed in the crystal, the light of the frequency inside PBG is confined in the line defect. Then, the light of the frequency of PBG propagates along the line defect. Therefore, such photonic crystal can be used as an optical waveguide. Such optical waveguide formed in the photonic crystal is referred to as a line defect waveguide.
If an optical filter and the optical waveguide are formed, either of them or a combination thereof can compose an optical functional element such as an optical modulator and an optical switch. Main optical function elements can be formed in the photonic crystal, and the optical functional elements are connected to compose an optical circuit. From these reasons, the photonic crystal is expected as a platform for optical integrated circuits.
At this point, in order to use the effect of PBG in the three directions of x, y, and z which are vertical to each other, it is required for the refractive index distribution of the photonic crystal to have three-dimensional periodic structure. However, the three-dimensional periodic structure is complicated, and thereby increasing the manufacturing cost. Thus, the photonic crystal (hereinafter may be referred to as a “two-dimensional photonic crystal”) in which the refractive index distribution has two-dimensional periodic structure is often used. To be specific, the two-dimensional photonic crystal is used in which the refractive index distribution has periodicity on a substrate surface, but has a limited thickness with no periodicity in the thickness direction. In that case, light confinement in the thickness direction of the substrate is realized by total reflection caused by a refractive index difference and not the effect of PBG.
However, the characteristics of the two-dimensional photonic crystal with limited thickness do not completely match the characteristics of the two-dimensional photonic crystal with unlimited thickness. However, if the refractive index distribution in the thickness direction of the two-dimensional photonic crystal with limited thickness is reflectionally symmetric in the area where light propagates, they almost match the optical characteristics of the two-dimensional photonic crystal with unlimited thickness. Operation prediction of a device by the two-dimensional photonic crystal with unlimited thickness is much easier than operation prediction considering over the limited thickness. Then, if the two-dimensional photonic crystal having reflectional symmetric refractive index distributions can be used, design of the device using the photonic crystal can be made easy as well.
There are some specific structures realized so far as the two-dimensional photonic crystal with limited thickness. Among them, a pillar type tetragonal lattice photonic crystal has a characteristic that propagation speed of light in a line defect waveguide is slow in a wide band. That is, it is slow group velocity. Generally, if a waveguide with slow propagation speed is used, an optical circuit of the same function can be made by a shorter waveguide length. Therefore, the line defect waveguide using the pillar type tetragonal lattice photonic crystal is suitable for the optical integrated circuit.
FIG. 9 is a pattern diagram illustrating the configuration of a line defect waveguide of the pillar type tetragonal lattice photonic crystal with limited thickness. In the illustrated pillar type tetragonal lattice photonic crystal, cylinders 52a and 52b are disposed in a tetragonal lattice pattern in a low dielectric constant material 51. The cylinders 52a and 52b are cylinders with limited height made of high dielectric constant material. Further, the cylinder 52b has a smaller diameter than the cylinder 52a. Since the state in which these cylinders 52a and 52b are disposed in the tetragonal lattice pattern resembles the state in which atoms are disposed in a lattice pattern in crystals such as silicon and quartz and it is for an optical use, it is referred to as a “photonic crystal”. Therefore, the material of the low dielectric constant material 51 and the cylinders 52a and 52b are not necessarily crystal, and may be amorphous.
In the case of the photonic crystal illustrated in FIG. 9, the cylinder 52a is a complete photonic crystal cylinder, whereas the cylinder 52b has a smaller diameter than the cylinder 52a. Thus, the cylinder 52b is considered as a defect introduced in a perfect crystal. In the following explanation, in order to distinguish the perfect crystal cylinder 52a from the cylinder 52b which is equivalent to a defect, the former may be referred to as a “non-line defect pillar” and the latter may be referred to as a “defect pillar”, a “defect cylinder”, or a “line defect pillar”. However, it should be noted that the line defect pillar itself does not especially have a defect.
The photonic crystal line defect pillars 52b illustrated in FIG. 9 are arranged to form a line on a certain straight line. Accordingly, a line defect waveguide is formed by the line defect pillars 52b and the surrounding non-line defect pillars 52a. In the line defect waveguide of the cylinder type tetragonal lattice photonic crystal illustrated in FIG. 9, the line of the line defect pillars 52b is equivalent to a core in a waveguide of total reflection confinement type such as an optical fiber. Moreover, the lattice of the non-line defect pillars 52a on the both sides thereof and the surrounding low dielectric constant material 51 are equivalent to a clad. In the case of the total reflection confinement type waveguide, the total reflection confinement type waveguide functions as a waveguide only by the existence of the core and the clad. Similarly, in the case of the line defect waveguide, line defect waveguide functions as a waveguide only by the existence of the line defect pillar 52b, the surrounding non-line defect pillar 52a, and the low dielectric constant material 51.
The optical devices and the optical circuits using the pillar type tetragonal lattice photonic crystal are expected to be miniaturized and higher integrated. However, there have been no structure heretofore that effectively takes advantage of the usage of photonic crystals for a 2×2 optical switch focused in the present invention.
Now, there is an optical switch using a Mach-Zehnder interferometer by a waveguide as one of the relevant 2×2 optical switches. FIG. 10 is a pattern diagram illustrating the configuration thereof. The configuration and an operation of an optical switch 72 of FIG. 10 are as follows.
The optical switch 72 of FIG. 10 is composed of a 3 dB directional coupler 60, a 3 dB directional coupler 61, and a waveguide 62 and a waveguide 63 therebetween. Light entered from either one of an input port 70 and an input port 73 propagates through a waveguide 71 or a waveguide 74, and is incident on the 3 dB directional coupler 60. The 3 dB directional coupler 60 divides this incident optical power by half. The divided light respectively propagates through the waveguide 62 and the waveguide 63, and enters two waveguide 64 and waveguide 65 which compose the 3 dB directional coupler 61. Then, light is emitted from emission ports 66 and 67 via waveguides 68 and 69.
The ratio of the optical power emitted to the emission port 66 and the emission port 67 of the 3 dB directional coupler 61 changes by the relationship in phases between the light entered in the waveguide 64 and waveguide 65. Specifically, the ratio of the optical power emitted to the emission port 66 and the emission port 67 changes depending on which of the waveguides 64 and 65 has progressed or delayed how much phase between the light entered the waveguide 64 and the waveguide 65. By using this phenomenon, the exit of light can be switched between the emission port 66 and the emission port 67 by adjusting the phase difference of light while the light propagates through the waveguide 62 and the waveguide 63.
The adjustment of the phase difference while the light propagates through the waveguide 62 and the waveguide 63 is performed by changing an effective refractive index of only one waveguide, for example, the waveguide 63. The change of the effective refractive index is performed by changing the refractive index of the material of the waveguide using heat or an electric field.
The amount of change in the phase of the light propagating through the waveguide increases in proportion to the length of the waveguide. For this reason, the length of both of the waveguides, which are the waveguide 62 and the waveguide 63, is increased while keeping the length to be the same or only the length of the wavelength 63 is increased with the effective refractive index that is to be changed. This enables an easy operation of the optical switch by a small refractive index change of the waveguide material.
Note that the case in which the waveguide 62 and the waveguide 63 have the same length is referred to as a symmetrical Mach-Zehnder type 2×2 optical switch, and the case in which the waveguide 62 and the waveguide 63 have different length may be referred to as an asymmetrical Mach-Zehnder type 2×2 optical switch.
[Patent Document 1]
    Domestic Re-publication of PCT International Publication for Patent Application, No. 2005-085921